Social media erupt over US Biggest Loser

A day after Rachel Frederickson won the latest US season of The Biggest Loser, after shedding nearly 60 per cent of her body weight, attention wasn't focused on her $US250,000 ($A281,357) win - but rather the criticism surrounding her loss.

Experts cautioned that regardless of her current weight, the criticism being levied on social media about her losing too much isn't helpful.

A more constructive message is needed, they say, centring on overall healthy living and body image.

The 5-foot-4 (162cm) tall, 24-year-old Frederickson dropped from 118 kg under the show's rigorous exercise and diet regimen, and time spent on her own before the finale.

She was a three-time state champion swimmer at Stillwater Area High School in Minnesota, then turned to sweets for solace after a failed romance with a foreign exchange student she followed to his native Germany.

Frederickson's newly thin frame lit up Twitter on Wednesday, with many viewers pointing to the surprised expressions on the faces of trainers Jillian Michaels and Bob Harper during the show's Tuesday night finale.

Many tweeted that Frederickson looked anorexic and unhealthy, while others congratulated her for dropping 155 pounds (70 kg).

Jillian Lampert, senior director of the Emily Program, an eating disorder treatment program based in St Paul, said Frederickson's body mass index is below the normal range.

But she said the criticism directed against her isn't helpful.

"As a society we often criticise people for being at higher weights - that's part of why we have the TV show The Biggest Loser - and then we feel free to criticise lower weight," Lampert said.

A more constructive message to send young people would centre on well-rounded health and the importance of eating well, moving well and sleeping well, she said.

"We certainly see a lot of people who struggle with eating disorders who use the same behaviours on that show to an extreme," she said.

"That can't be helpful."

Joanne Ikeda, a dietitian and retired faculty member at the University of California at Berkeley's Department of Nutritional Sciences, added that focus needs to be on embracing body-size diversity.